1. Field of the Invention
This invention relates to a spatial optical modulator device for effecting modulation of a optical wave surface. In particular, the present invention relates to a structure and method of driving a fine electrode liquid crystal device type spatial optical modulator device.
2. Description of the Related Art
To fix and control an optical wave surface, a diffraction plate using a photographic method, a diffraction grating using mechanical machining of an optical lens has been widely used in the past, while an acoustico-optical device and a transparent piezoelectric device such as a PLZT have been used to effect control that changes with time. A precision stationary optical wave surface modulation device fabricated in the course of time can be utilized for time-fixed control of the optical wave surface, but a pattern updating time of the optical wave surface modulation device must be as fast as up to about tens of milli-seconds for moving pictures. It is possible to mechanically switch a large number of holograms that have been taken in advance and reproduce them as moving pictures, as in motion pictures. However, devices and techniques suitable for updating the pictures on a real time basis for imaging and reproducing actual moving pictures have been unavailable. In contrast, various studies of moving diffraction gratings have been made in the past, but none of them have been entirely satisfactory. For example, a phase modulator device of an optical wave surface for forming a pattern by an electron beam on an oil film and controlling the film thickness requires large scale vacuum tubes and exhaust systems, and the life of the oil film is short so this method is not easy to apply. A system that scans a smectic liquid crystal using a laser beam has a resolution of up to micrometers, but since a thermal write system is employed, a long time is necessary for heat transfer and heat equilibrium. In other words, about one second is necessary for drawing one screen, and this drawing speed is not sufficient.
Liquid crystal video display devices that have been developed for the display of liquid crystal television receivers and personal computers have low response speed of 20 msec and 50 msec because they are designed to correspond to moving pictures. From the viewpoint of response speed and pixel density, liquid crystal devices for display are effective devices, and the feasibility of using the liquid crystal display device as a diffraction grating has been examined in the past. However, according to an estimation of the practical size limits of the liquid crystal pixels, the lower limit is regarded as being some tens of microns. In the case of an STN (super-twist: a liquid crystal device having a twist angle of 120.degree. to 270.degree., for example) with a liquid crystal layer having a thickness of 5 .mu.m, the minimum dimension of each of the widths of the pixel electrode and the gap between the pixel electrode is 5 .mu.m, that is, the pixel pitch is about 10 .mu.m and even in the case of ferroelectric liquid crystal devices, the limit is about 4 .mu.m, and the resolution is 100 to 250 (lines/mm). A higher resolution has been believed impossible in view of the limit of the thickness of the liquid crystal layer.
The present invention provides a method of correctly realizing shape accuracy and positional accuracy of a diffraction pattern to be formed, by precisely controlling a group of liquid crystal molecules as a function of a location near a fine synthetic electric field which is generated by combining pixels, which are smaller than the thickness of a liquid crystal layer, and gaps between the pixels.
The present invention discloses a liquid crystal cell structure, orientation and material necessary for realizing this device structure, and application examples effectively utilizing the features of the diffraction device according to the present invention.
Hereinafter, the conventional optical wave surface modulator device described above, and the structure of the conventional liquid crystal display device will be explained with reference to the drawings.
Conventionally, the size of a pixel of a liquid crystal display device used for personal computers and word processors has been determined on the premise that one pixel can be visually identified as one dot by most people. Therefore, the dimensions of a pixel in the liquid crystal device has been established to be 250 to 350 .mu.m, the gap between the pixels is about 10% of the pixel dimension, that is, from 25 to 35 .mu.m, in order to obtain an aperture ratio of at least 80% and to improve production yield. In the reproduction of a video image, the pixels themselves need not be discriminated, but the screen becomes easier to view if the pixels are not distinctive. For this reason, devices having a pixel dimension of 100 to 200 .mu.m have been used. Since optical enlargement is effected in a liquid crystal view finder used for monitoring an image of a video camera and in a liquid crystal device of a projection type display device, devices having a pixel dimension of 30 .mu.m and a pixel gap of 5 to 8 .mu.m have been produced tentatively. These devices have resolution of some dozens of lines per milli-meter, but this resolution is not sufficient for use in a diffraction device. Moreover, when the pixel dimension is reduced, the aperture ratio of the pixel becomes smaller in proportion to the square of the dimensional ratio. According to a conventional design concept, if the width of the pixel electrode and the electrode gap are set to 18 .mu.m and 6 .mu.m, respectively, to secure an aperture ratio of at least 50% when the thickness of the liquid crystal layer is 5 .mu.m, resolution is about 40 lines/mm, and this value is by far lower than the level necessary for holography, that is, from hundreds to thousands of lines per millimeter. Furthermore, when the electrode pitch is reduced, the aperture ratio of the pixel drastically decreases, the resolution of at least some tens of lines per millimeter is believed unrealistic. Accordingly, the pursuit of higher resolution has been abandoned.
The problems to be solved by the present invention reside in the accomplishment of pattern formation of an optical wave surface modulator device for electronic control on a real-time basis by the use of a liquid crystal diffraction device with a higher level of accuracy. More specifically, the present invention is directed to the following points:
a) to form a precision modulation pattern for suppressing a spatial frequency noise component of a diffraction pattern, and to form a modulation pattern of a fine dimensional region below approximately the thickness of a liquid crystal;
b) to accomplish precision pattern positional accuracy by suppressing a positional error of a diffraction pattern; and
c) to avoid a drop in an aperture ratio when the fine diffraction pattern is formed.